Page:EB1911 - Volume 20.djvu/291

 leather, except the opening left in each, which is closed by the key-valve or pallet.

The sliders are connected with the draw-stops or stop-handles, which are covered in with stout upper boards, on which the pipes stand. The stop-handles are pulled out, and holes are then bored straight down through the upper boards, sliders and table to admit the wind from the grooves to the pipes. When the sliders are shifted by pushing in the handles, the holes no longer correspond, and the pipes are silenced.

Pipes are divided first into flue-pipes and reed-pipes. Flue-pipes are blown by a wind mouthpiece characteristic of the organ, while in reed-pipes the wind acts on a metal tongue vibrating on a reed, and the motion of the tongue determines the speech of the pipe.

Pipes are made either of wood or of metal. Wood flue-pipes are generally of the form of a rectangular parallelepiped, metal flue-pipes of a cylindrical shape. Reed-pipes are conical or pyramidal, and widen towards the top. Some flue-pipes are made with stopped ends; these as a rule sound a note about an octave lower than the corresponding open pipes of the same length. Such are the stopped diapason, bourdon, and stopped flute.

The general elementary theory of the resonance of a pipe is tolerably simple. The effective length of the pipe is determined by measuring from the upper lip to the open end in open pipes, and from the upper lip to the stopper and back again in stopped pipes. To this is added an allowance for the effect of each opening, since the condition of perfect freedom from constraint does not subsist at the opening itself. The corrected length is traversed twice (backwards and forwards) by sound, in the time of one vibration of the resultant note. This describes in a rough and general manner the way in which any disturbance gives rise to the note of the pipe; but the theory of the mouth-pieces is a much more difficult matter, into which we cannot here enter.

In reed-pipes which are simply conical the resonance of the body is nearly the same as that of an open pipe of the same length. Where the form is irregular no simple rule can be given. But the resonance of the body of the pipe is generally the same as the note produced. The tongue of a reed-pipe alternately opens and closes the aperture of the reed. In this way it admits pulses of wind to the body of the pipe; these, if they recur at the proper intervals, maintain its vibration, which takes place when the note produced corresponds to the resonance of the pipe. The reed itself has its vibrating length determined by a wire which presses against it. The free end of this wire is touched with the tuning tool until a satisfactory note is produced.

The pitch of the different stops is commonly denoted by the conventional approximate length of the pipe sounded by C, the lowest key of the manual. Even in incomplete stops which have no bass, the length of the pipe which C would have if the stop were extended down serves to indicate the pitch.

The conventional length of the C-pipe for stops having the normal pitch of the keys is 8 ft.; a pipe having twice this length sounds the octave below, a pipe having half that length the octave above, and so on. Thus stops which sound the octave below the normal pitch of the keys are spoken of as 16-foot stops. Even where the pipes are stopped so that the actual length is only 8 ft., they are spoken of as having “16-ft.tone.” Similarly 32-ft. stops sound two octaves below the normal pitch of the keys. But if these notes are produced by stopped pipes, whose actual length is only 16 ft., they are spoken of as having “32-ft. tone.” Sixteen-foot and 32-ft. stops are specially characteristic of the pedal, where the names also signify the length of the open pipe which would sound the note actually produced by the lowest C. Of stops higher than the normal pitch of the keys, the octave is denoted by 4 ft. if made with open pipes, 4-ft. tone if stopped; the twelfth is commonly spoken of as 2, the fifteenth or double octave as 2 ft. Higher-sounding stops are occasionally used, but these generally form part of “mixtures,” and the foot-lengths of the separate ranks are not usually given.

The true or accurate lengths of the pipes vary within considerable limits. The base of the scales (dimensions) varies according to the standard of pitch, and the voicing and the complicated natural laws of pipes produce other deviations from simple relations, so that the conventional dimensions can only be regarded as a simple means of classifying the stops according to their pitch-relations. For this purpose they are essential; they are continually appealed to in discussion and description; and they are almost invariably marked on the stop-handles in all countries, so that a moderate knowledge of foreign nomenclatures, combined with the habit of seizing the meaning of the figures such as 16, 8, 4, on the stop-handles, will frequently suffice as a key to the complexities of a foreign organ.

Each of the manuals, or rows of keys, of an organ constitutes a separate organ, which is more or less complete in itself. The names of the different manuals or organs are great organ, swell organ, choir organ and solo organ. The fifth manual, where it occurs, is the echo organ. The above is the usual order in point of development and frequency of occurrence, although the solo is sometimes preferred to the choir organ. The great organ is in a certain sense the principal department of the organ. It may be regarded as formed by a completely developed series of those fundamental stops which constitute the solid basis of the tone of the instrument. If an instrument be constructed with only a single manual this necessarily assumes, in general, the characteristics of a great organ. The great organ is called “grande orgue” in French, and first manual or “haupt-werk” in German.

It is proposed to describe the principal organ-stops under the